It has hitherto been known to achieve the above objective with an optical encoder. Optical encoders normally used for this purpose have a round rotating glass/Perspex disc having ‘n’ graticules on its periphery and a marker on one side and dull black background. It is fixed on the shaft. An opaque rectangular disc with three windows (one window in front of marker, and two windows having graticules with mark/space ratio corresponding to rotating disc) is mounted on the static surface in front of rotating disc with its marking face towards round disc with minimum spacing between the discs. The lines on both discs are evenly spaced, and the line width is equal to the spacing between adjacent lines. A light source (LED) is placed at the back of stationary plate. The light reflected back through the graticule window area of the static plates is detected by the photosensor. When the lines on the two discs coincide, minimum amount of light is reflected. When the lines on One disc are fully over transparent portion of second disc, maximum amount of light will be reflected. Accordingly as rotor disc moves relative to stator disc, a sine type waveform is generated by the photosensor. Moire fringes shadow patterns as shown in FIG. 1(A) are formed in this manner.
The frequency of sine waves is proportional to number of graticules ‘n’ on the rotating disc/revolution of shaft. Value of ‘n’ can be 50, 100, 250, 360, 500 and 540. Some commercial available encoders are listed in Table 1.
The main shortcoming of existing method is that the maximum possible graticules/mm for a small encoder (17.2 mm diameter disc) is limited to about ‘Ten’ where proper alignment with other disc is achievable. This results in about 540 graticules per circle resulting in 540 pulses/revolution. The maximum count frequency is limited to 200 KHz only.